1. Field of the Invention
The invention relates to a digital circuit including an interpolation filter for over-sampling a digital input signal in the form of a temporal sequence of samples and a noise shaper having a quantizer responsive both to the output of the interpolation filter and to feedback due to its own output.
2. Description of the Prior Art
Such a digital circuit arrangement is known from DE-PS 30 31 012. In this known arrangement the samples of the digital input signal, which have a specific sampling rate, are applied to an interpolation filter comprising a register. The output of the register is connected to a noise shaper, comprising a quantizer, two adder stages and one filter. The output signal of the filter is subtracted from the output signal of the register in the first adder stage. The output signal of the first adder stage is applied to the quantizer, which is a linear quantizer. In a linear quantizer the quantization steps and the intervals between the individual quantization steps are of equal magnitude. The second adder stage forms the quantization-error signal between the output signals of the quantizer and of the first adder stage is formed and supplies it to the filter arrangement. The frequency with which the samples of the quantization-error signal are applied to the filter arrangement is higher than the frequency with which the samples of the input signal are loaded into the register. The noise shaper is followed by a digital-to-analog converter whose analog output signal is applied to a low-pass filter which suppresses periodic spectra of the useful signal and which effects timeaveraging.
The last sample read appears on the output of the interpolation filter employed in DE-PS 30 31 012 and in the following noise shaper it is further processed with a higher frequency. Thus, this interpolation filter effects oversampling.
A digital circuit arrangement known from the magazine "Elektronik" No. 14 of 15.07.1983 pages 61 to 64 comprises an interpolation filter in which the samples of the input signal are read in at a rate of 44.1 kHz and read out at a rate of 176.4 kHz. This interpolation filter provides oversampling by a factor of 4 as well as low-pass filtering to suppress the periodic spectra of the useful signals. The interpolation filter is followed by a noise shaper whose output signal is applied to a digital-to-analog converter. The noise shaper comprises a first order filter.
The analog signal obtained by digital-to-analog conversion of the quantized digital signal contains quantization noise caused by said quantization. The digital signal applied to the interpolation filter and the noise shaper is further quantized in these two circuits by reduction of the word length, which gives rise to additional quantization noise. This additional quantization noise, produced in the noise shaper, is less than the quantization noise arising by quantization in a conventional quantization circuit without oversampling. The extent to which the quantization noise is reduced depends on the choice of the filter. The energy of the quantization noise is partly transferred from the signal-frequency range to a higher frequency range. The reduction of the quantization noise in the signal frequency range then depends on the filter order. The filter must be a filter which is optimized for a specific order for which the power density spectrum of the quantization noise is minimal. On the other hand, the complexity increases with the order of the filter. A second-order filter provides a satisfactory compromise between complexity and the extent to which the quantization noise of the filter is reduced.
Practical tests with second-order noise shapers have shown that for higher values of the input signal an instability of the digital circuit arrangement occurs, which impairs the desired noise shaping.
3. Object of the Invention
It is the object of the invention to construct a digital circuit arrangement of the type defined in the opening paragraph in such a way that no instability occurs.